The developers of "Minecraft" are constantly updating and improving elements of the game, so even though it is constantly changing, a user only needs to pay for the game once. As of 2015, there is only one computer version of "Minecraft" to buy.
There are also versions of "Minecraft" on other platforms, including the Xbox 360 and Xbox One editions, the Playstation 3 and 4 editions, and "Minecraft: Pocket Edition" for tablets and phones. All versions are constantly being updated and improved and only require the player to pay the initial fee.
Many "Minecraft" players like to augment the game by installing user-created mods. There are many to choose from, and they are all free to download.